Rolled ferrite-pearlite alloy plate and method of processing same

ABSTRACT

THIS INVENTION RELATES TO A FERROUS ALLOY AND A METHOD FOR IMPROVING THE STRENGTH AND NOTCH TOUGHNESS OF A FERROUS ALLOY CONTAINING BY WEIGHT UP TO ABOUT 0.35% CARBON, A TOTAL UP TO ABOUT 3% OF OTHER ELEMENTS, BALANCE IRON. MORE PARTICULARLY, SAID INVENTION IS PREDICATED ON THE DISCOVERY THAT BY CONTROLLING THE REDUCTION ASPECTS OF THE CONTINUOUS THERMAL MECHANICAL TREATMENT SCHEDULE THROUGH THE AUSTENITE, AUSTENITE-FERRITE, AND FERRITE REGIONS, A HIGHLY TEXTURED FERROUS ALLOY RESULTS HAVNG IMPROVED PROPERTIES.

My 30, 1974 5, F1 MELLQY ETAL 3,826,691

ROLLED FERRITE-PEARLITE ALLOY PLATE AND METHOD OF PROCESSING SAME Filed Feb. 5, 1973 E w .2 M D E E m w 5 m 5 e Z w w w 2 -0 .la 00 00 00 w @w 0 2 /6 22 be Mask QQWQSHE M w a w M m s m w w l H 6m 6 W 47 2 a Q 4 4 b L L[ 0 O 0 0 O o m w a w w w m w M a K WGSK wkwaimk United States Patent 3,826,691 ROLLED FERRITE-PEARLITE ALLOY PLATE AND METHOD OF PROCESSING SAME George F. Melloy, Bruce L. Bramfitt, and Arnold R.

Marder, Bethlehem, Pa., assignors to Bethlehem Steel Corporation Filed Feb. 5, 1973, Ser. No. 329,473 Int. Cl. C21d 7/14; C22c 39/00 U.S. Cl. 148-12 6 Claims ABSTRACT OF THE DISCLOSURE REFERENCE TO RELATED APPLICATIONS The invention herein represents a modification and/or improvement over the inventions disclosed in application Ser. No. 786,844, filed Dec. 20, 1968, now U.S. Pat. 3,645,801, issued Feb. 29, 1972, and application Ser. No. 156,215, filed June 24, 1971, as a division of said first application.

BACKGROUND OF THE INVENTION In the study of the mechanism of thermomechanical treatment or rolling of a ferrous alloy containing by weight, up to about 0.35% carbon, a total up to about 3% of other elements, and the balance iron, various parameters were investigated to determine their effect on strength and notch toughness. The preliminary aspects of this study culminated in U.S. Cl. Pat. No. 3,645,801 assigned to the assignee hereof.

The thermal mechanical treatment described in said patent, hereinafter referred to as continuum rolling, covers a method wherein a ferrous alloy, whose chemistry is set forth above, is subjected to a rolling sequence as it cools from a temperature at which it is austenitic, where at least one reduction takes place in each of the regions; austenite, austenite-ferrite, and ferrite. The temperatures defining the transition between regions are the Ar and Ar temperatures.

Since a procedure resulted in a significant improvement in strength and notch toughness, the latter manifesting itself in a lower transition temperature. This was particularly true when the process was directly compared to the conventional hot-rolling practice, and a subsequent development called "low-finishing-temperature rolling. These practices are discussed in greater detail in said patent.

However, with all of said methods, it was believed that large reductions were essential to the achievement of improved properties. For example, with regard to the latter 7 prior art system, it has been said that the development of higher strength in the low-alloy steel by low-finishingtemperature rolling requires reduction of cross-sectional area of approximately 15% or more at the lower-thannormal temperature.

In contrast to this apparent limitation, it has now been determined that by increasing the number of passes during rolling, and by lowering the percent reduction per pass, it is possible to obtain a ferrous alloy plate with a higher degree of texture and lower transition temperature than obtained in the shorter and more severe continuum rolling schedule of U.S. Pat. 3,645,801. This is achieved without any sacrifice on strength and other mechanical properties. Finally, an added commercial advantage to the longer or extended pass schedule is the reduction in mill load requirements. For instance, mill loads are reduced by up to 40%.

SUMMARY OF THE INVENTION Briefly, in the practice of this invention, the method comprises heating a ferrous alloy containing, by weight, a maximum of 0.35% carbon, a total up to about 3% of other elements, balance essentially iron to above the Ac critical temperature, effecting a reduction of said alloy in one or more passes by an amount less than about 12.5% per pass as it cools, effecting one or more reductions of said magnitude as the alloy is cooling in the range between the Ar and Ar, critical temperatures, and continuing said reduction schedule within the temperature range between the Ar critical temperature and about 600 F. In the as-reduced condition, said ferrous alloy is characterized by high strength, good notch toughness, and a preferred crystallographic texture dominated by a cube-on-corner of (111) orientation.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1a is a graph showing a representative cooling curve during the thermal mechanical treatment of a ferrous alloy according to this invention, the circles representing the temperatures of rolling of the alloy.

FIG. 1b is a graph showing a cooling curve during thermal mechanical treatment for a 20% reduction schedule of a ferrous alloy treated according to the process of U.S. Pat. No. 3,645,801, the squares representing the temperatures of rolling of the alloy.

FIG. 1c is a graph similar to FIG. 1b except that the treatment follows a 30% schedule, the triangles representing the temperatures of rolling of the alloy.

The Ar and Ar temperatures were determined from the actual cooling curves in FIGS. la, 1b and 10, which in turn were constructed from temperature measurements during rolling. This method was used instead of dilatometric measurements since deformation affects the critical temperatures, particularly the ferrite and pearlite start temperatures, i.e. Ar;, and Ar DETAILED DESCRIPTION OF PREFERRED EMBODIMENT In the practice of this invention, and of the procedure set forth in U.S. Pat. No. 3,645,801, the thermomechanical treatment thereof differs from the conventional methods in that the ferrous alloy being treated is preferably rolled or reduced within three temperature ranges. Each of said ranges is characterized by a distinct crystalline structure or combination thereof. For example, the first reductions are effected above the Ar critical temperature where the alloy is fully austenitic. The next significant treatment is between the Ar and Ar critical temperatures Where the alloy is a combination of austenite and ferrite. The final treatment is between the Ar, critical temperature and about 600 F., within which the alloy is fully ferritic.

The method and the improvement set forth in detail later are based on the discovery that by putting work or energy through reductions, i.e. rolling, into the alloy, and avoiding complete recrystallization, such as more than 60%, after each reduction, a small but significant amount of energy is retained therein, thereby resulting in an increase in toughness and strength in the alloy. It has now been discovered that further improvements in toughness and texture are possible through a control on the thermal mechanical treatment schedule.

Patented July 30, 1974 v I To demonstrate the effectiveness of the improved procedurefa ferrous alloy whose chemistry, by weight, is set forth below: 1

*Nickel, molybdenum, each less than about 0.02% and siligon, ychromium, vanadium, tin, titanium, each less than about was subjected to the three thermal mechanical schedules whose cooling curves are shown in the figures. Specifically, a 500 lb. heat of said alloy was melted and from this a 9%" square ingot was cast. The ingot was slabbed into three 4 inch thick billets, with one each subjected to the rolling schedules of Table I.

TABLE I Schedule A (10% aim) Thickness Pcrcen (inches) reduction Pass number:

Cumulative reduction 197. 50

Schedule B aim) Thickness Percent (inches) reduction Pass number:

Cumulative reduction 185. 58

Schedule C aim) Thickness Percent; (inches) reduction P 5 number:

Cumulative reduction; 174

The cooling rates during the rolling of the respective slabs to /2 inch plates were essentially identical as evi- The three rolled plates were then examined for mechanical proerties, and the following results observed.

TABLE II Percent Y.S. T.S. along. Percent CV15 trans. Plato (k.s.'.) (k.s.i.) (2' RA. temp. F.)

A 62. 3 7l. 7 33. 5 75. 5 ll0 B 60. 1 72. 3 34. 5 76. l C 62. 0 74. 3 33. 0 74. 2 -50 While no significant changes were noted in the strength and ductility of the alloy plates, there was a dramatic lowering of the transition temperature, signifying an improvement in toughness.

After the metallurgical and crystallographic examination of the plates were concluded, it was determined that the change in toughness was attributed to the development of a cube-on-corner or (111) [110] texture, said texture being most pronounced in the plate rolled according to Schedule A. Table III shows the metallographic data and crystallographic texture thereof, the latter based upon the inverse pole figure method. On rolled plates, such as processed above, the texture is measured on the transverse plane, or perpendicular to the rolling direction. When processed according to Schedule A, the intensity of the [110] thereof should exceed about 3.5 times random as measured by the nine-plane inverse pole figure technique. A value of 1.0 represents a completely random orientation, whereas 9.0 shows or the orientation of a single crystal.

With this background, said data is presented below.

TABLE III Grain size Percent Percent recrystal- Plato in ASTM pearlite lization intensity It is clear from the preceding that the intensity of the [110] (cube-on-corner) texture was much stronger in the plate rolled in accordance with Schedule A. And, as stated earlier, it is believed that the improvement in toughness is attributed to the development of this texture. Without desiring to be bound by theory, the development of this preferred texture is the result of increasing the cumulative percent reduction in the regions containing ferrite, namely, the temperature region below the An, critical temperature. For example, from FIG. 1a under Schedule A, passes 7-21 covered a cumulative reduction of 137.71%, while from FIGS. 1b and 10 under Schedules B and C, the cumulative reductions for passes 4-9 and 3-6 were 123.70% and 116.00%, respectively. This theory is supported by the proposition noted earlier in regard to the fractional retention of energy by the workpiece following each reduction. Greater amounts of energy are retained in the workpiece subjected to the longer schedule; thus, said retained energy is more effective in developing the desired texture. That is, the development of the desirable cube-on-corner texture is a result of the dynamic recovery process where a fully polygonized substructure forms. This texture has little effect on strength but the notch toughness is enhanced since it depends upon cleavage plane orientation.

In any case, since the critical reductions are those occurring after the ferrous alloy has cooled below the Ar;,, larger initial reductions could be imposed on the plate. However, it is critical to the achievement of the results hereof to limit all reductions below the Ar to a maximum of about 12.5%, preferably at least a majority thereof to a maximum of about 10.0%.

By so limiting the reduction schedule for the ferrous alloy of this invention, further advantages are gained.

Specifically, a significant drop in the mill load was noted for Schedule A. For example, the mill loads at the last pass, for Schedules A to C, respectively, were as follows: 34,800 lb./in., 49,800 l./in., 68,500 lb./in. This latest recognition suggests that what may have been impossible for some mills, has now been made possible by the present invention.

These and other advantages will become apparent to those skilled in the art, particularly after reading these specifications. Accordingly, no limitation or restriction is intended to be imposed herein except as set forth in the appended claims.

We claim:

1. In an improved method of rolling a ferrous alloy containing, by weight, not more than about 0.35% carbon, up to a total of 3.0% of other elements, balance iron, to improve the strength and toughness thereof, wherein said ferrous alloy is heated to its austenitizing temperature and thereafter as it is being cooled subjected to a plurality of cross-section reducing operations between the Ar and Ar temperatures, and a plurality of said cross-section reducing operations between the Ar, and about 600 F., the improvement comprising in combination therewith the provision that each said reducing operation results in a reduction of the ferrous alloy by an amount no greater than about 12.5%, and that said rolled ferrous alloy is characterized by a preferred crystallographic texture.

2. The method according to claim 1 wherein the majority of said crosssection reducing operations are by an amount no greater than about 10.0%.

3. The method according to claim 1 including the step of initially reducing the cross-section of the ferrous alloy at a temperature above the Ar 3 4. The method according to claim 1 wherein said preferred crystallographic texture consists of the (111) orientation, where the intensity thereof exceeds about 3.5 times random as measured by the nine-plane, inverse pole figure technique.

5. The method according to claim 2 wherein about 15 cross-section reducing operations occur below the Ar critical temperature.

6. A rolled ferrite-pearlite alloy plate exhibiting a combination of high strength and good toughness and containing, by weight, not more than about 0.35% carbon, up to a total of 3.0% of other elements, balance iron, said alloy plate being characterized by a preferred crystallographic texture dominated by the (111)[110] orientation, where the intensity of said texture exceeds about 3.5 times random as measured by the nine-plane, inverse pole figure technique.

References Cited UNITED STATES PATENTS WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 14836 

